Conventionally, for example, in a continuous galvanizing line, excessive molten zinc is blown off by causing pressurized air or gas to be ejected from an air knife portion (for example, one configured by using an air nozzle) onto a steel sheet that travels while passing through and being drawn up from a molten zinc bath, thereby achieving a desired plating thickness. In such a case, if the steel sheet vibrates in the direction in which it moves closer to and away from the air knife portion, the distance between the nozzle and the steel sheet will fluctuate, resulting in fluctuation of the pressure (ejection force) to which the steel sheet is subjected, which causes the thickness of plating to become non-uniform, leading to deterioration of quality.
Accordingly, an electromagnetic vibration suppression device has been contrived which controls the current to be applied to electromagnets which are oppositely disposed at locations interposing the traveling steel sheet, and controls the attractive force of the electromagnets, thereby reducing the vibration of the traveling steel sheet (for example, Patent Literature 1). An electromagnetic vibration suppression device of this kind is configured such that a plurality of electromagnet pairs including electromagnets oppositely disposed in the thickness direction of the steel sheet are provided in the width direction of the steel sheet, and further, a plurality of displacement sensors for detecting a relative position (distance) with respect to the steel sheet are also provided in the width direction of the steel sheet in a combination associated with each electromagnet, so that the current to be applied to each electromagnet is controlled based on the relative position (distance) with respect to the steel sheet which is detected by each displacement sensor.
Incidentally, a steel sheet that travels between opposing electromagnets may meander in the width direction. Since the edge (end edge) position of the steel sheet changes in front of and behind of the location where meandering has occurred, a control specification for adjusting the output current to each electromagnet according to the change of the edge position is required.
Accordingly, Patent Literature 1 discloses an embodiment including displacement sensors disposed at positions opposable to the edge position of the steel and other sensors (edge position detection sensors) disposed at a predetermined pitch in the width direction of the steel sheet so as to constantly detect the edge position of the traveling steel sheet to determine whether or not the steel sheet is present by each edge position detection sensor so that when the presence of the steel sheet is detected, an electromagnet associated with the edge position detection sensor is driven and, on the other hand, when the presence of the steel sheet is not detected, the driving of the electromagnet associated with the edge position detection sensor is stopped.
Further, an attempt has been made to increase the rigidity of the steel sheet itself during conveyance, by conveying the steel sheet while keeping it curved in the width direction. In this case, the curved shape of the steel sheet on which appropriate vibration suppression control can be exerted by an electromagnetic vibration suppression device is predefined, and the electromagnetic vibration suppression device is configured such that the output current to electromagnets making up each electromagnet pair is controlled such that the steel sheet takes a desired curved shape.